STITCH-2298 Swift SDK: Todo app for mobile sync

[jsflax]

I would recommend checking out the branch and viewing the files that way– the RWLock changes make the diff a bit difficult to read.

• Change RWLock to use GCD
• Fix the way updateDescription is calculated for replaces
• Sanitize documents before updateDescription is calculated
• Add retry logic for ChangeStreamListeners
• Add ToDo App

[jsflax]

@adamchel PTAL